ROT13 (Caesar Cipher)

ROT13 (“rotate by 13 places“, sometimes hyphenated ROT-13) is a simple substitution cipher used in online forums as a means of hiding spoilers, punchlines, puzzle solutions, and offensive materials from the casual glance. ROT13 has been described as the “Usenet equivalent of a magazine printing the answer to a quiz upside down”.ROT13 is an example of the Caesar cipher, developed in ancient Rome. (Wikipedia)

Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary. A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence continues at the beginning of the alphabet: N becomes A, O becomes B, and so on to Z, which becomes M. Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English alphabet and 26 = 2 × 13, the ROT13 function is its own inverse. (Wikipedia)

Write a function that takes a string and returns the ROT13 version of the string; you may assume that the character set is ascii. What is the meaning of “Cebtenzzvat Cenkvf vf sha!” .

The transition table for rot13:

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

E

F

G

H

I

J

K

L

M

Examples of transitions:

The Wall

Gur Jnyy

Smoke on the water

Fzbxr ba gur jngre

Initially I wanted to resolve the challenge in a functional programming language. Given the fact that my Haskell skills are very low, I’ve tried to write a functional approach in python . The results… lesser readability, fewer lines of code & fewer minutes: